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Ancient sawfish fossil sheds light on tooth evolution

Detailed 3D X-ray images of the saw-like nose of Schizorhiza stromeri challenge the classical theory that vertebrate teeth evolved from external scales.

A new study led by Museum scientists, published in Proceedings of Royal Society B, has found evidence disputing the long-established theory that all teeth developed from external scales.

Using a micro-CT scanner to analyse a 70-million-year-old fossil of Schizorhiza stromeri (an extinct ray with a protruding, saw-like nose), the team found significant differences between the ancient saw-teeth and the teeth in the mouths of other closely related cartilaginous fish such as sharks.

The Museum researchers, along with collaborators from the University of Sheffield, Birkbeck University and the University of Vienna, say their results could prompt a re-think of the evolutionary history of teeth for all vertebrates.

Schizorhiza stromeri nose fossil with teeth highlighted

The fossil nose of Schizorhiza stromeri, with the saw-teeth highlighted


Cutting through

The 1.5-metre-long nose (or rostrum) of S. stromeri  was brought to the Museum in January 2015, having been discovered by miners excavating phosphate-rich rock in northern Morocco.

The ancient ray, which roamed the ocean between 72 and 66 million years ago, had hundreds of arrow-headed tooth-like structures lining its rostrum.

These saw-teeth, which are actually modified scales rather than teeth, formed two cutting blades. The ray used them to slash through shoals of small fish, sucking up the dead and injured.

Reconstruction of Schizorhiza stromeri

Reconstruction of Schizorhiza stromeri © C. Underwood


Museum palaeobiologist Dr Zerina Johanson, lead author of the NERC-funded study, says the apparent similarities between the saw-teeth of S. stromeri and the teeth of other vertebrates (especially sharks) made the fossil a good test case for the standard theory of tooth evolution.

'The classical theory says that teeth developed from external structures in the skin, such as scales, which, through evolution, moved into the mouth.

'This theory would predict that these skin structures and teeth should be very similar, and we argue that this includes important features of teeth such as organisation and replacement - think of the ongoing replacement of teeth in the shark mouth,' says Johanson.

Rows of teeth in a nurse shark jaw

Shark teeth - such as these ones belonging to a nurse shark - typically grow in rows, unlike the stacked saw-teeth of Schizorhiza stromeri. By Luca Oddone [CC BY-SA 3.0], via Wikimedia Commons.


Close inspection

The team studied the S. stromeri rostrum using the Museum's micro-CT scanner, revealing minute details hidden within the fossil.

They discovered that the structure and organisation of the tooth-like scales were actually very different to the oral teeth of most vertebrates, casting doubt on the supposed developmental link between the two.

First, a series of replacements grew directly beneath the fully grown saw-teeth. This differs significantly to sharks, whose replacement teeth usually come forward in rows and are not typically stacked directly on top of each other.

Second, the X-ray images revealed the unusual rotations the saw-teeth underwent while growing. New saw-teeth were produced at the very tip of the rostrum, but they passed through two 90-degree rotations before taking their place between previously formed roots.

'The unusual rotation, as well as the positioning directly beneath existing saw-teeth, is very different from typical teeth, which generally show a much simpler type of rotation into functional position,' says Johanson.

CT scan showing tooth rotation of Schizorhiza stromeri

The detailed scans showed how replacement saw-teeth slowly rotated into position


The scientists say these results are not what would be expected on a conventional understanding of tooth evolution, which holds that teeth evolved from hardened external scales and gradually moved into the mouth.

'We think our results question the classical theory, which covers vertebrates as a whole,' says Johanson.

The team now plan to use the same methods to research a range of fossil and living vertebrates, and find out more about the evolution and diversity of the teeth on some of the most dangerous creatures on our planet.